Structures and methods for reducing corrosion in wire bonds

ABSTRACT

A semiconductor structure includes a bond pad and a wire bond coupled to the bond pad. The wire bond includes a bond in contact with the bond pad. The wire bond includes a coating on a surface of the wire bond, and a first exposed portion of the wire bond in a selected location. The wire bond is devoid of the coating over the selected location of the wire bond, and an area of the first exposed portion is at least one square micron.

BACKGROUND

1. Field

Embodiments of this disclosure relate to packaged semiconductor devices.More particularly, this disclosure relates to devices and methods forreducing corrosion in wire bonds.

2. Related Art

Wire bonding is used in the semiconductor industry to form electricalconnections between contact pads on an integrated circuit (IC) die andcontact pads on a package substrate. Ball bonds are formed with one endof the wire on the contact pads of the IC die and wedge or stitch bondsare formed with the other end of the wire on the contact pads of thepackage substrate. Alternatively, wedge bonds can be used on both endsof the wire at the bond pad on the IC die and the bond pad on thepackage substrate.

In some instances, the wire is coated with one or more layers of ametal, ceramic and/or organic material that exhibit desirablecharacteristics such as promoting formation of a bond during the wirebonding process and improving reliability in air-to-air temperaturecycling and high temperature bake. The coating can also help preventcorrosion of the wire when the packaged semiconductor device duringassembly and when used in the field. A problem can arise if portions ofthe coating are removed during the bonding process thereby exposing thewire to potentially corrosive elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 is a top view of an embodiment of semiconductor device includingan integrated circuit die wire bonded to a package substrate prior to anencapsulation process.

FIG. 2 is a cross-sectional side view of a wire bonded IC as depicted inFIG. 1.

FIG. 3 is a cross-sectional side view of an embodiment of a ball bondformed during a bonding process.

FIG. 4 is a side view of the ball bond of FIG. 3 after the bondingprocess and intentionally removing a portion of coating on the wire.

FIG. 5 is a side view of an embodiment of a stitch bond of FIG. 1 afterbonding and intentionally removing a portion of coating on the wire.

DETAILED DESCRIPTION

Devices and methods disclosed herein help minimize or reduce corrosionin wire bonds by removing coating to expose a portion of a wire in aselected location. The location selected is generally in the vicinity ofareas of the wire bond that were unintentionally exposed during thebonding process. Corrosion of the metal wire may be observed when thepackage undergoes high temperature bake such as 175° C. for longduration such as 1000 hours. In situations where the coating is a metalmore noble than the underlying wire metal, the exposed areas cause theformation of an electrochemical corrosion cell, whereby the metalcoating acts as a cathode, and the exposed underlying metal wire portionacts as anode from which portions of the wire metal will be removed, orcorrode. When the unintentionally exposed areas are small compared tothe area of the remaining portion, then the relatively small anode areacauses corrosion to occur quickly. By selectively removing additionalportions of the coating material, the anode area is increased relativeto the cathode area, and the corrosion rate at each exposed locations issignificantly slower. In addition, the selectively exposed area helpsattract corrosive agents that would otherwise be attracted to theunintentionally exposed areas, potentially causing a deeper level ofcorrosion that could impair conductivity or even break the wire. Byselectively exposing areas of the wire, corrosion may still occur, butthe corrosion level is not likely to be as extensive, thereby promotingreliability and integrity of the wire bond.

FIG. 1 is a top view of an embodiment of semiconductor device 100including an integrated circuit die 104 wire bonded to a packagesubstrate 102 prior to an encapsulation process. The die 104 includesbond pads 106 for electrical access to the functional circuitry of thedie. The bond pads 106 may be made of aluminum, aluminum alloyed withcopper and/or silicon, aluminum on copper, or other suitablematerial(s). The package substrate 102 includes bond pads 108 for makingelectrical connections to the substrate 102. The bond pads 108 may bemade of gold, gold on nickel, gold on palladium, gold on copper, silveror other suitable material(s). Wire bonds 112 including a respectiveball bond 116 on each of bond pads 106 and stitch bond 114 on each ofbond pads 108 also include a connecting wire 110 between ball bond 116and stitch bond 114. The wire bond 112 may be made of gold, gold alloyedwith other elements, copper, aluminum, silver or other suitablematerial(s). The package substrate 102, for example, may be a mechanicalsupport for the bond pad 108 or may include electrical circuitry such asrouting lines and conductive interconnects, some of which may beelectrically coupled to bond pad 108. For example, package substrate 102can be a leadframe, or ball grid array substrate made of epoxy, plastic,FR-4, FR5, a Bismaleimide-Triazine resin, a fiberglass reinforced epoxylaminate, polytetrafluorethylene, ceramic, polyimide, or other suitablematerial.

FIG. 2 is a cross-sectional side view of a wire bonded IC as depicted inFIG. 1. Wire 110 is bonded to a bond pad 106 and pad 108 to form anelectrical connection between the integrated circuit 104 and the packagesubstrate 102. Although a wire bond 112 is depicted and the descriptionrefers to a wire bond as a concrete example, other bonding arrangementsmay be substituted for the wire bond.

FIG. 3 is a cross-sectional side view of an embodiment of the formationof ball bond 116 on bond pad 106 using a wire bond process. In theexemplary structure depicted in FIG. 3 wire 110 is made from copper or acopper alloy and a coating 300 of a layer of palladium, or gold andpalladium. In other embodiments, wire 110 may be formed of material suchas copper, copper alloy, silver, silver alloy, gold, gold alloy,aluminum or aluminum alloy, or other suitable conductive material orcombination of materials. Coating 300 may be made of one or morematerials such a metal, a ceramic, or organic material, or combinationsthereof. The coating 300 is formulated to prevent or at least minimizecorrosion and oxidation of the underlying wire 110 and may also exhibitother desirable characteristics such as promoting formation of the bondduring the wire bonding process and high reliability in air-to-airtemperature cycling and high temperature bake. The coating mayadditionally or alternatively be formulated to provide electricalinsulation to the wire 110 such that when the wires 110 come intocontact with each other, they do not electrically short to each other.

Wire 110 is held in a bonding tool referred to as capillary 302 while afree air ball (FAB) (not shown) at the end of wire 110 is formed byelectrical flame-off that heats the end of wire 110 to a malleablestate. The FAB is then lowered to contact bond pad 106, compressed, andsubjected to ultrasonic generation (USG). The vibration of the USGeffectively scrubs the FAB against the aluminum bond pad 106, promotinginterdiffusion of the metal of the FAB and the metal of the bond pad106, creating a conductive intermetallic compound 312. At the same time,portions 306, 308, 310 of wire 110 may be exposed where coating 300 isunintentionally removed by a surface 304 of the capillary 302 rubbingagainst coating 300 during the bonding process. A chamfered area wherethe capillary 302 contacted the ball bond 116 remains after the wirebonding process.

The device 100 (FIG. 1) is typically encapsulated in a mold compoundonce wire bonds 112 are formed. One problem that can arise when one ormore relatively small areas of coating 300 are unintentionally removedis that corrosive elements such as adhesion promoters that can containsulfur or sulfate from the mold compound are attracted to the underlyingwire 110. The smaller the area exposed, the deeper the corrosion mayextend into wire 110, eventually affecting the ability of wire 110 toconduct electricity reliably.

In situations where the coating 300 is a metal more noble than theunderlying metal in wire 110, the exposed areas of wire 110 cause theformation of an electrochemical corrosion cell, whereby the metalcoating 300 acts as a cathode, and the exposed underlying metal wire 110acts as anode from which portions of the wire 110 will be removed, orcorrode.

FIG. 4 is a side view of the ball bond 116 of FIG. 3 after the bondingprocess and after intentionally removing a portion 404 of coating 300 onthe wire 110. In the example shown, exposed portions 306-310 remain andother unintentionally exposed portions 402 on the exterior surface ofball bond 116 are shown. Intentionally exposed portion 404 of wire 110is formed by removing more of coating 300 in a selected location.Portion 404 of coating 300 can be removed using one or more suitableprocesses including removal using laser 406 capable of emitting a laserbeam 408, a chemical, and/or a mechanical device. Intentionally exposedportion 404 can have any suitable shape such as a stripe, a geometricshape, and/or an irregular shape. A minimum size or area for exposedportion 404 can be specified, for example, greater than or equal to onesquare micron.

The location of exposed portion 404 is typically chosen to be closeenough to unintentionally exposed portions 306-310, 402 to attractcorrosive elements away from exposed portions 306-310, 402, or at leastdiffuse the corrosive elements over a larger area of exposed portions306-310, 402 and 404 to prevent the corrosion from extending to a levelthat would affect the reliability of wire bond 112. Also, theintentionally exposed area 404 increases the total anode area relativeto the cathode area, reducing the anodic corrosion rate caused by theoverall electrochemical cell. In some embodiments, the intentionallyexposed portion 404 can be located to include one or more of theunintentionally exposed portions 306-310, 402, thereby incorporating theone or more unintentionally exposed portions 306-310, 402 in theintentionally exposed portion 404.

Additional intentionally exposed portions having the same or differentsize and shape may be formed in suitable locations relative tounintentionally exposed portions 306-310, 402. Further, an intentionallyexposed portion may be formed on a section of wire 110 that is not partof ball bond 116.

FIG. 5 is a side view of an embodiment of a stitch bond 114 of FIG. 1after bonding and intentionally removing a portion 502 of coating 300 onwire 110. Stitch bond 114 can also include unintentionally exposedportions 504 of wire 110 that were formed during the bonding process.Intentionally exposed portion 502 of wire 110 is formed by removing moreof coating 300 in a selected location. Portion 502 of coating 300 can beremoved using one or more suitable processes including removal usinglaser, a chemical, and/or a mechanical device. Intentionally exposedportion 502 can have any suitable shape such as a stripe, a geometricshape, and/or an irregular shape. The location of exposed portion 502 istypically chosen to be close enough to unintentionally exposed portions504 to attract corrosive elements away from exposed portions 504, or atleast diffuse the corrosive elements over a larger area of exposedportions 504 and 502 to prevent the corrosion from extending to a levelthat would affect the reliability of wire bond 112. Also, theintentionally exposed area 502 increases the total anode area relativeto the cathode area, reducing the anodic corrosion rate caused by theoverall electrochemical cell. In some embodiments, the intentionallyexposed portion 502 can be located to include one or more of theunintentionally exposed portions 504, thereby incorporating the one ormore unintentionally exposed portions 504 in the intentionally exposedportion 502.

Additional intentionally exposed portions having the same or differentsize and shape may be formed in suitable locations relative tounintentionally exposed portions 504. Further, an intentionally exposedportion may be formed on a section of wire 110 that is not part ofstitch bond 114.

It should be understood that in this description, as in the artgenerally, materials referred to by their elemental names may containtrace impurities and/or be alloyed with small amounts of othermaterials. For example, the wire material commonly referred to as“copper” in the art may be alloyed with aluminum or silicon or both insmall amounts, such as a few percent by weight or less, to modifycertain properties of pure copper.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent invention. Any benefits, advantages, or solutions to problemsthat are described herein with regard to specific embodiments are notintended to be construed as a critical, required, or essential featureor element of any or all of the claims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thusthese terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

I/we claim:
 1. A method comprising: coupling a wire bond to a bond pad,wherein the wire bond includes a coating, and the wire bond is incontact with the bond pad; and subsequent to the coupling the wire bond,removing a first portion of the coating on the wire bond in a selectedlocation.
 2. The method of claim 1, wherein the wire bond includes oneor more exposures in the coating, and the one or more exposures are inproximity to the first portion.
 3. The method of claim 2, wherein theone or more exposures exist in the coating prior to the removing thefirst portion of the coating, and the one or more exposures arelocalized within a contact area of the wire bond that has been incontact with a bonding tool during a wire bonding process.
 4. The methodof claim 3, wherein the selected location is located outside of thecontact area.
 5. The method of claim 1, wherein the removing the firstportion of the coating utilizes one of laser removal of the coating,mechanical removal of the coating, and chemical removal of the coating.6. The method of claim 1, wherein the wire bond includes one or moreadditional coatings, and the removing the first portion of the coatingincludes removing a second portion of the one or more additionalcoatings.
 7. The method of claim 1, wherein the wire bond includes abond in contact with the bond pad, and the first portion includes aminimum width of one micron.
 8. The method of claim 1, wherein the wirebond includes one of copper, silver, and aluminum, and the coatingincludes one of palladium-inclusive coating, organic coating, andceramic coating.
 9. The method of claim 1, wherein the wire bondincludes a ball bond in contact with the bond pad.
 10. The method ofclaim 1, wherein the wire bond includes a stitch bond in contact withthe bond pad.
 11. A semiconductor structure comprising: a bond pad, anda wire bond coupled to the bond pad, wherein the wire bond includes abond in contact with the bond pad, the wire bond includes a coating on asurface of the wire bond; and a first exposed portion of the wire bondin a selected location, wherein the wire bond is devoid of the coatingover the selected location of the wire bond, and an area of the firstexposed portion is at least one square micron.
 12. The semiconductorstructure of claim 11, wherein the wire bond includes one or moreexposures in the coating in proximity to the first exposed portion ofthe wire bond.
 13. The semiconductor structure of claim 12, wherein theone or more exposures are located within a wire bonding tool contactarea of the wire bond.
 14. The semiconductor structure of claim 12,wherein the selected location is located outside of a wire bonding toolcontact area.
 15. The semiconductor structure of claim 12, wherein theone or more exposures are located outside of a wire bonding tool contactarea.
 16. The semiconductor structure of claim 12, wherein the selectedlocation is located within a wire bonding tool contact area.
 17. Thesemiconductor structure of claim 11, wherein the wire bond furtherincludes one or more additional coatings, the semiconductor structurefurther includes a second exposed portion of the wire bond, and the wirebond is devoid of the one or more additional coatings over an area ofthe second exposed portion.
 18. The semiconductor structure of claim 11,wherein the semiconductor structure further includes a plurality of wirebonds and a plurality of exposed portions, each of the plurality of wirebonds includes at least one of the plurality of exposed portions, andthe plurality of exposed portions are aligned along an edge of thesemiconductor structure.
 19. A semiconductor structure comprising: aplurality of bond pads aligned along an edge of the semiconductorstructure, and a plurality of wire bonds coupled to the plurality ofbond pads, wherein each of the plurality of wire bonds includes a bondin contact with one of the plurality of bond pads, and each of theplurality of wire bonds includes a coating on a respective wire bondsurface; and a plurality of exposed portions within a selected area,wherein the selected area spans across the plurality of wire bonds in adirection along the edge of the semiconductor structure, each of theplurality of wire bonds includes at least one exposed portion of theplurality of exposed portions, and each of the plurality of wire bondsis devoid of the coating over the at least one exposed portion.
 20. Thesemiconductor structure of claim 19, wherein the semiconductor structurefurther includes one or more additional exposed portions, wherein thewire bond is devoid of the coating over the one or more additionalexposed portions.